Process for preparing silicon hydrides

ABSTRACT

In preparing of silicon hydrides with use of a mixture of an alkyl aluminum hydride and a trialkyl aluminum as a reducing agent for a silicon compound, an aluminum halide compound is added to said mixture in an amount sufficient for converting at least 90 mol % of the trialkyl aluminum to a dialkyl aluminum monohalide prior to the reduction reaction. Silicon hydrides are obtained with a high yield and quality.

FIELD OF THE INVENTION

This invention relates to a process for the preparation of siliconhydrides, particularly monosilane, disilane and derivatives thereof withhigh yield and reduced amount of by-products.

BACKGROUND OF THE ART

It has been conventionally known to prepare monosilane, disilane orderivatives thereof by using an alkyl aluminum hydride. For example, inJapanese patent application Publication No. 36-517, British Pat. No.823,483, German Pat. Nos. 1,055,511 and 1,117,090, there is disclosed aprocess for preparing a silicon hydride by the reduction of a siliconhalide with sodium hydride in the presence of an alkyl aluminum hydride.In this known process, it is sodium hydride that serves as reducingagent for the silicon halide, and the alkyl aluminum hydride is added insuch a small amount as 0.1-30% so that it serves only as an activatingagent.

Another process for the reduction of a silicon halide is disclosed inFrench Pat. No. 1,499,032, in which the reducing agent consists of analkyl aluminum hydride only.

These processes, however, are not yet satisfactory in that the yield ofend products is lower and high quality is not obtained.

From the industrial standpoint, it is more advantageous to utilize analkyl aluminum hydride in the form of a mixture of said alkyl aluminumhydride and a trialkyl aluminum rather than as pure alkyl aluminumhydride, since such a mixture is commercially available at a low price.However, when the reduction of a silicon halide is conducted using sucha mixture of alkyl aluminum hydride and trialkyl aluminum, there is anextremely low yield of a silicon hydride, together with a large amountof by-produced silicon halide hydride, for example, monochlorosilaneSiH₃ Cl, which is probably produced because of insufficient reduction ofthe silicon halide. In addition, there is produced a large amount ofethane as by-product.

An object of the present invention is to provide a process for thepreparation of silicon hydrides with improved yield and reduced amountof by-products without lowering the activity of alkyl aluminum hydrideby using a mixture of an alkyl aluminum hydride and a trialkyl aluminumwhich is easily available as the reducing agent for silicon halides.

DISCLOSURE OF THE INVENTION

In accordance with the present invention there is provided a process forthe preparation of silicon hydrides by reducing a silicon compound witha mixture of an alkyl aluminum hydride and a trialkyl aluminum, whichcomprises adding preliminarily into said mixture an aluminum halidecompound in an amount sufficient for converting at least 90 mol percentof the trialkyl aluminum to a dialkyl aluminum monohalide prior to thereduction reaction.

Surprisingly, it has been found that when adding a specified amount ofthe aluminum halide compound to the mixture of alkyl aluminum hydrideand trialkyl aluminum prior to the reduction of the silicon compound,silicon hydrides of high yield are obtained without reducing theactivity of alkyl aluminum hydride and by-producing of silicon halidehydride and ethane is extremely reduced.

BEST MODE FOR CARRYING OUT THE INVENTION

The starting silicon compound which may be used in the present inventionis a compound of the formula Si_(n) X_(2n+2) in which n is an integerequal to or larger than 1 and the X's may be the same or different andrepresent halogen atoms, hydrogen atoms, alkoxy groups, alkyl groups,aryl groups or vinyl groups, the case of all X's being hydrogen atomsbeing excluded. The larger n becomes, the more difficult it is to handlesuch silicon compound and therefore to work the present invention.

Typical groups or atoms as X include methyl group, ethyl group, propylgroup, n-butyl group, iso-butyl group, pentyl group, vinyl group, phenylgroup, p-methylphenyl group, methoxy group, ethoxy group and halogenatoms such as chlorine and bromine. As the preferred compound fallingwithin the above-mentioned general formula for use in the presentinvention there are included such compounds as silicon tetrachloride,hexachlorodisilane, tetraethoxysilane, diethyldichlorosilane ortrichlorosilane.

The alkyl aluminum hydride for use in the present invention is areducing compound having the general formula R₂ AlH. The symbol Rrepresents an alkyl group having 1 to 10 carbon atoms including ethylgroup, n-propyl group or iso-butyl group. The preferred compoundsexpressed by said general formula include such compounds as diethylaluminum hydride or diisobutyl aluminum hydride.

The trialkyl aluminum to be used in the present invention is a compoundexpressed by the general formula R₃ Al. The symbol R represents an alkylgroup having 1-10 carbon atoms, for example, ethyl group, n-propyl groupor iso-butyl group. As preferred compounds of the formula, there can beexemplified such compounds as triethyl aluminum and tri-isobutylaluminum.

It is often the case, with a mixture of an alkyl aluminum hydride andtrialkyl aluminum, which is available commercially at a low price, thatthe alkyl groups of the alkyl aluminum hydride and the trialkyl aluminumare the same. However, for the process of the present invention, amixture can be used in which the alkyl groups of the alkyl aluminumhydride and the trialkyl aluminum are different. The mixture of alkylaluminum hydride and trialkyl aluminum is generally available in aproportion of 3:7 to 8:2 (by weight ratio).

The aluminum halide compound for use in the present invention is acompound expressed by the general formula AlR_(n) X_(3-n). X representshalogen atoms such as chlorine or bromine. R represents alkyl groupshaving 1-10 carbon atoms such as ethyl group, n-propyl group oriso-butyl group. However, the presence of alkyl group is not alwaysnecessary, and thus, n is a number selected from 0, 1 and 1.5. Aspreferred compounds, there can be exemplified such compounds as ethylaluminum dichloride, ethyl aluminum sesquichloride, iso-butyl aluminumdichloride or aluminum chloride. A mixture of such compounds can also beused in the present invention. The alkyl group of the alkyl aluminumhalide to be added to the mixture of alkyl aluminum hydride and trialkylaluminum may be the same as the alkyl groups of each component of saidmixture. Otherwise, the alkyl groups of these three components may bedifferent from one another.

The dialkyl aluminum monohalide formed from the conversion of thetrialkyl aluminum may be expressed by the general formula AlR₁ R₂ X. R₁and R₂ are the same or different and represent alkyl groups having 1 to10 carbon atoms such as ethyl group, n-propyl group or iso-butyl group.X represents halogen atoms such as chlorine or bromine. As the compoundsof the formula there can be exemplified such compounds as diethylaluminum monochloride or di-isobutyl aluminum monochloride.

According to the present invention, the aluminum halide compound must beadded to the mixture of alkyl aluminum hydride and trialkyl aluminum insuch an amount as to convert at least 90 mol percent of the trialkylaluminum to the dialkyl aluminum monohalide. This amount to be added isdetermined by the amount of trialkyl aluminum in the mixture and theamount of halogen atoms present in the aluminum halide compound. Forexample, in case of using alkyl aluminum dihalides or alkyl aluminumsesquihalides, these must be added in an amount of at least 90 mol % ofthe trialkyl aluminum present in the mixture. In case of aluminumchloride, this must be added in an amount of at least 45 mol % of thetrialkyl aluminum.

According to the present invention, the aluminum halide compound isreacted with the trialkyl aluminum to form the dialkyl aluminummonohalide as shown hereunder:

    R.sub.3 Al+AlRX.sub.2 →2AlR.sub.2 X

    R.sub.3 Al+Al.sub.2 R.sub.3 X.sub.3 →3AlR.sub.2 X

    2R.sub.3 Al+AlCl.sub.3 →3AlR.sub.2 Cl

Namely, the alkyl aluminum dihalide or alkyl aluminum sesquihalide arereacted with the chemical equivalent of the trialkyl aluminum to formthe dialkyl aluminum monohalide, while aluminum chloride is reacted with1/2 equivalent of the trialkyl aluminum to form the dialkyl aluminummonohalide. For this reason the aluminum halide compounds must be addedin the amount of at least 90 mol % or at least 45 mol %, respectively.The maximum amount of the aluminum halide compound to be added is notlimited. However, even addition of such compound in a larger amount willprovide no particular effects. Rather, too much addition of suchcompound is not preferred since it leads to the reduction of theconcentration of said compound and an increase in the total volume ofthe reaction system, which result in commercial disadvantages such asdecrease in the reaction velocity, increase in the required volume ofthe reaction vessel or increase in required heat supply.

The addition of the aluminum halide compound to the mixture of alkylaluminum hydride and trialkyl aluminum may be accomplished by admixingthe former directly with the latter. Otherwise, it may be done bydiluting each component with a solvent and then admixing the resultants.In almost all cases, there will evolve heat due to admixing, and hence,care must be taken to avoid excessive heating. If one or more of thecompounds to be admixed is solid, such compound may be dissolved in asolvent or suspended in a suitable medium, prior to the additionreaction. In the latter case where the compound is suspended, heating ispreferable, in order to accelerate the reaction.

According to the present invention, a silicon hydride is produced by thereduction reaction of the silicon compound as herein defined with themixture of alkyl aluminum hydride and trialkyl aluminum, which has beenadded with the aluminum halide compound, in the manner, for example, asexpressed by the equation,

    SiCl.sub.4 +4Al(C.sub.2 H.sub.5).sub.2 H→SiH.sub.4 +4Al(C.sub.2 H.sub.5).sub.2 Cl

or

    Si(OC.sub.2 H.sub.5).sub.4 +4Al(C.sub.2 H.sub.5).sub.2 H→SiH.sub.4 +4Al(C.sub.2 H.sub.5).sub.2 (OC.sub.2 H.sub.5)

Such reduction reaction may be carried out without any solvent. However,the use of a solvent is generally preferable for assuring that thereaction proceeds moderately. The solvent is preferably the one thatwill neither react with alkyl aluminum compounds present in the reactionsystem nor form aluminum complex, so that the alkyl aluminum compoundscan be recovered after the reduction reaction. Preferable solvents arealiphatic hydrocarbons or aromatic hydrocarbons such as heptane, octane,liquid paraffin, benzene or toluene. A polar solvent, such as diethylether or tetrahydrofuran, may be used, if desired, to accelerate thereduction reaction at a lower temperature. However, such a polar solventwill form a complex salt with alkyl aluminum compounds, and hence, it isextremely difficult to separate and recover the alkyl aluminum compoundfrom the reaction system. It is impossible to make such separation bymeans of such a simple procedure as distillation. It should be furthernoted that the use of a solvent such as carbon tetrachloride must beavoided, since it will react with alkyl aluminum compounds.

The reduction reaction will not proceed properly if the temperature istoo low, while there will occur decomposition of the alkyl aluminumcompounds and formation of side reactions at excessively hightemperatures. Thus, the reduction reaction may be carried out generallyin the range of -30° C. to 100° C., more preferably in the range of 0°C. to 50° C. The reduction reaction will proceed sufficiently under apressure in the range of atmospheric pressure to 2 Kg/cm² (gauge).However, the reaction may also be carried out under higher or lowerpressures, depending upon the temperature or the reactor employed.

The reactants and the products of the reduction reaction are all activeand, most of them will decompose or ignite by the reaction with oxygenor water. Thus, reaction must be conducted under an atmosphere where thereactants and the products are kept inactive with oxygen and water, forexample, under the atmosphere of an inert gas (e.g. helium or argon),nitrogen or hydrogen which has been sufficiently removed of oxygen andwater contents.

The reduction reaction may be carried out in any manner, i.e.,batch-wise, semibatch-wise or continuously.

According to the process of the present invention, there can be obtainedat a lower cost concurrently a silicon hydride and an alkyl aluminummonohalide in the following manner, which has been hitherto otherwisevery expensive: An alkyl aluminum hydride, a trialkyl aluminum and analkyl aluminum halide, which have the same alkyl groups, are subjectedto the reduction reaction, in such a proportion that the amount of thesealkyl groups to the amount of halogen atoms present in the siliconhalide (as raw material) and alkyl aluminum halide be 2:1 by mol. Then,following the separation of a silicon hydride (the main product) fromthe reaction system, there can be very easily recovered from theremaining reaction system a single dialkyl aluminum monohalide, forexample, by means of distillation.

According to the present invention, there is produced silicon hydride,such as monosilane, disilane, diethyldihydrosilane or the like. Thesesilanes are of commercial importance since they have a variety ofapplications including uses as raw materials for synthesizing organiccompounds or inorganic compounds, fuels or catalysts. The monosilaneprepared by the process of the present invention contains minimal amountof impurities, and hence, through a simple refining procedure, serves asa raw material for various types of semiconductors.

The invention will be more fully described by reference to the followingexamples, but, the examples are not for restricting the scope of theinvention.

EXAMPLE I

A stainless steel autoclave, having a volume of 500 ml and provided withan induction stirring device, was connected, via a gas flow meter, witha stainless steel vessel for trapping gas. The autoclave was alsoconnected with a pump for feeding silicon tetrachloride dissolved inliquid paraffin therein. The whole system was filled with helium gas,prior to the reaction. The gas-trapping vessel was kept cool by means ofliquid nitrogen.

A mixture (49.2 g) of 70% by weight of diethyl aluminum hydride and 30%by weight of triethyl aluminum was dissolved in 50 g of liquid paraffin.The amounts of the diethyl aluminum hydride and the triethyl aluminumwere 0.4 mols and 0.129 mol, respectively. There was dissolved 16.4 g(0.129 mols) of ethyl aluminum dihalide in 23 g of liquid paraffin, andthe resultant solution was added drop-wise to the above-mentionedmixture. As will be noted, the amount of the ethyl aluminum dichlorideused is the one to convert the total mols of the triethyl aluminum todiethyl aluminum monochloride.

The mixture of the three alkyl aluminum compounds was placed in a vesseland maintained for 20 minutes at 30° C. under the pressure of 2 mmHg fordegasification. Then, the mixture was placed into the autoclave under ahelium atmosphere. Meanwhile, 17.0 g (0.1 mols) of silicon tetrachloridewas dissolved in 28 g of liquid paraffin, and the resulting solution wasforced, by means of the pump, into the autoclave, which was kept at 40°C., over a 2-hour period. The monosilane gas produced was caught in thetrapping vessel. After the reaction was completed, the remainingmonosilane was pressed out by helium to be caught in the trappingvessel.

The composition of the gas trapped was determined by means of gaschromatography. It was found that the production of the monosilane gaswas 2.04 l (under normal condition) and the yield thereof was 91%. Theratio of ethane produced to the monosilane (ethane/SiH₄) and that ofmonochlorosilane to the monosilane (SiH₃ Cl/SiH₄) were found to be 0.15%and 50 ppm, respectively.

COMPARATIVE EXAMPLE I

The same procedure as in Example I was repeated, except that ethylaluminum dichloride was not added. The production and the yield ofmonosilane were 0.29 l (under normal condition) and 13%, respectively.Ethane/SiH₄ and SiH₃ Cl/SiH₄ were found to be 1.5% and 2.1%,respectively.

EXAMPLE II

The reduction of tetraethoxy silane was performed in the same procedureas in Example I with the same amounts of the alkyl aluminum compounds asin Example I, except that 70 ml of toluene, which had been sufficientlyremoved of water and oxygen, was used as a solvent, in place of theliquid paraffin, and that 20.8 g (0.1 mols) of tetraethoxy silanedissolved in 30 ml of toluene (removed of water and oxygen) was used inplace of the silicon tetrachloride dissolved in the liquid paraffin. Theperiod of the reaction was 1.3 hours, and the reaction temperature was20° C.

The production of monosilane gas was 1.97 l (under normal condition) andthe yield thereof was 88%. Ethane/SiH₄ was found to be 0.3%.

COMPARATIVE EXAMPLE II

The same procedure as in Example II was repeated, except that ethylaluminum dichloride was not added. The production of monosilane was 1.1l (under normal condition) and the yield of the same was 49%.Ethane/SiH₄ was found to be 1.1%.

EXAMPLE III

The reduction reaction was conducted with the same apparatus and thesame materials as in Example I. There was placed in the autoclave 17.0 gof silicon tetrachloride dissolved in 28 g of liquid paraffin. To amixture of 49.2 g of diethyl aluminum hydride and triethyl aluminum wasadded drop-wise 16.4 g of ethyl aluminum dichloride dissolved in 23 g ofliquid paraffin. The mixture of the three aluminum-containing compounds,after having been degasified, was fed into the autoclave by the pumpover a 2-hour period. The reaction temperature was 40° C.

The production and the yield of monosilane were 1.90 l (under normalcondition) and 85%, respectively. It was found that ethane/SiH₄ was0.18% and SiH₃ Cl/SiH₄ was 1200 ppm.

EXAMPLE IV

A mixture (75.7 g) of 75% of diisobutyl aluminum hydride and 25% oftriisobutyl aluminum was dissolved in 50 g of n-heptane which had beenremoved of water and oxygen. The amounts of the di-isobutyl aluminumhydride and the triisobutyl aluminum were 0.4 mols and 0.096 mols,respectively. To the mixture, there was added drop-wise 15.6 g (0.1mols) of isobutyl aluminum dichloride dissolved in 20 g of n-heptane.The amount of the dichloride used corresponds to that for converting the104% of the triisobutyl aluminum to diisobutyl aluminum monochloride.The mixture of the three alkyl aluminum compounds was placed in theautoclave (500 ml) under an atmosphere of helium. Meanwhile, there wasprovided 17.0 g (0.1 mols) of silicon tetrachloride dissolved in 30 g ofn-heptane. The subsequent procedure was the same as in Example I.

The production of monosilane was found to be 2.00 l (normal condition)while the yield thereof was found to be 89%. The ratio of butaneproduced to the monosilane (butane/SiH₄) was 0.16% and SiH₃ Cl/SiH₄ was200 ppm.

EXAMPLE V

A mixture (49.2 g) of 75% by weight of diethyl aluminum hydride and 30%by weight of triethyl aluminum was diluted with 50 g of liquid paraffin.To the resultant mixture was added drop-wise 16.4 g of ethyl aluminumdichloride (which corresponds to the amount for converting all thetriethyl aluminum to diethyl aluminum monochloride) dissolved in 23 g ofliquid paraffin. The mixture of the three alkyl aluminum compounds wasintroduced in the autoclave of 500 ml kept under an atmosphere ofhelium. Then, 15.7 g of ethyl dichlorosilane dissolved in 30 g of liquidparaffin was added. The reaction was conducted at 40° C. for one hour.The liquid phase product was analyzed by means of gas chromatography,showing that the yield of diethyl silane was 95%.

EXAMPLES VI, VII, VIII AND COMPARATIVE EXAMPLES III, IV

The influence of the variation in the amount of ethyl aluminumdichloride on triethyl aluminum was studied when it was added to amixture of diethyl aluminum hydride and triethyl aluminum.

The apparatus and the procedure used were the same as in Example I,except that the mixture was the one of 50% by weight of diethyl aluminumhydride and 50% by weight of triethyl aluminum.

The results were shown in Table I, in which the conditions for theexperiments were also shown.

It can be seen from this Table that, for satisfactory results of thereduction reaction, ethyl aluminum dichloride should be added in such anamount as to convert at least 90 mol percent of triethyl aluminum todiethyl aluminum monochloride.

                                      TABLE I                                     __________________________________________________________________________             ##STR1##      (molar %)A*                                                                         (%)monosilaneYield of                                                               ##STR2##                                                                           ##STR3##                              __________________________________________________________________________    Comparative                                                                           0.5           50    17.5  3.25 18500                                  Example III                                                                   Comparative                                                                           0.7           70    35.5  0.43 1300                                   Example IV                                                                    Example VI                                                                            0.9           90    89.5  0.13  65                                    Example VII                                                                           1.1           100   91.0  0.12  60                                    Example VIII                                                                          4.0           100   80.0   0.202                                                                              200                                   __________________________________________________________________________     ##STR4##                                                                 

EXAMPLES IX, X AND COMPARATIVE EXAMPLES V, VI

The influence was studied of the amounts of aluminum chloride ontriethyl aluminum when the reduction was carried out using a mixture ofdiethyl aluminum hydride and triethyl aluminum.

The apparatus used and the synthetic method were the same as in ExampleI, except that the mixture consisted of 50% by weight of diethylaluminum hydride and 50% by weight of triethyl aluminum, and furtherthat the reaction of aluminum chloride with said mixture was carried outat 70° C. by dissolving the reactants in liquid paraffin.

The results were shown in Table II, from which it can be seen that, forsatisfactory results of the reduction reaction, aluminum chloride isnecessary to be added in such an amount to convert 90 mol % or more oftriethyl aluminum to diethyl aluminum monochloride.

                                      TABLE II                                    __________________________________________________________________________             ##STR5##      (molar %)B*                                                                         (%)monosilaneYield of                                                               ##STR6##                                                                           ##STR7##                              __________________________________________________________________________    Comparative                                                                           0.10          20    15    1.7  19500                                  Example V                                                                     Comparative                                                                           0.30          60    27    0.6  1800                                   Example VI                                                                    Example IX                                                                            0.45          90    89    0.25  20                                    Example X                                                                             0.60          100   91    0.24  25                                    __________________________________________________________________________     ##STR8##                                                                 

INDUSTRIAL APPLICABILITY

As described above, according to the process for the preparation ofsilicon hydrides by the present invention, end products of high qualityare obtained with high yield without reducing the activity of alkylaluminum hydride and the amount of by-products such as monochlorosilaneand ethane is reduced. There is a great utility in that a mixture of analkyl aluminum hydride and a trialkyl aluminum which is commerciallyavailable easily is used as the reducing agent for reduction of siliconhalides.

We claim:
 1. A process for the preparation of silicon hydrides byreducing a silicon compound of the formula, Si_(n) X_(2n+2) in which nis an integer equal to or larger than 1 and the X's may be the same ordifferent and represent halogen atoms, hydrogen atoms, alkoxy groups,alkyl groups, aryl groups or vinyl groups, the case of all X's beinghydrogen atoms being excluded; with a mixture of an alkyl aluminumhydride and a trialkyl aluminum, said alkyl aluminum hydride having theformula R₂ AlH in which R is an alkyl group of 1-10 carbon atoms; saidtrialkyl aluminum having the formula R₃ Al in which R is an alkyl groupof 1-10 carbon atoms; which comprises; adding to said mixture analuminum halide compound of the formula, AlR_(n) X_(3-n) in which nrepresents 0, 1, or 1.5, X represents halogen atoms, and R represents analkyl group having 1-10 carbon atoms, in an amount sufficient to convertat least 90 mol percent of the trialkyl aluminum to a dialkyl aluminummonohalide prior to the reduction of said silicon compound.